1. Field of the Invention
The present invention relates to a method of attaching tubes of a first material (typically a high-quality material, such as titanium or stainless steel) in a tube plate of a second material which is not weldable, or substantially not weldable, to the first material.
In heat exchangers and condensers where there are stringent demands for leak tightness in the attachment of the tubes in the tube plates, it is common to carry out such attachments by means of welding. This implies a requirement that there be good weldability between the materials constituting the tubes on the one hand and tube plates on the other hand. In many cases, for example in condensers, a corrosive fluid (i.e. liquid or gas) circulates within the tubes and in such use situations there is much in favour of constructing the tubes and tube plates of the same material, or at least of very closely related materials, to minimize any electropotential differences that arise and thus to reduce the risk of accelerated corrosion occurring.
2. Description of the Prior Art
In the past it has been known to use solid tube plates or tube plates of carbon steel, weld with a suitable corrosion-resistant material, for example with explosion-clad plates of the combination titanium/carbon steel, which are used for condensers with welded-in titanium tubes.
It has been established empirically that tubes of brass or similar material do not meet the requirements for corrosion resistance for, for example, condensers operating with a chemically aggressive cooling water. It may therefore be necessary to replace these tubes with tubes of titanium or tubes of a high-alloy stainless steel (austenitic or ferritic) so that the tubes will better withstand the corrosion. However, using these tubes gives rise to an attachment problem. Cooling water must not leak into the shell side of the condenser or other heat exchanger. This may occur in the case of damage to the tubes or in the case of a leaking tube/tube plate attachment.
In nuclear power plants, there is a stringent demand on a low chloride content in the condensate. Corrosion can be avoided by the use of a high-quality material for the tubes which, because of the cost of the high quality material, are usually made as thin-walled as safety conditions allow. However, it is difficult to attach thin-walled tubes securely enough to meet the high demands for fluid tightness if the tubes are attached merely by rolling them into the tube plate holes, and it is especially difficult to rebuild an old condenser by means of rolling new, high-quality tubes, into existing tube plates.
In copending U.S. patent application Ser. No. 619,366, filed on June 11, 1984, to R. Henriksson (assigned to the assignee of this application), the above-mentioned problems have been solved by a method which involves inserting the tube ends through the tube plates in such a way that each tube end extends somewhat beyond the outside of the respective plate. Thereafter, a cover plate provided with holes and being of the same material (or a similar material) to that of the tubes is laid over the outside of each tube plate and is attached to the tube ends by means of welding. A certain clearance gap might be left between the cover plate and the tube plate, thus providing a space between the two where leakage monitoring can easily be carried out.
However, this method presupposes the existence of projecting tube ends. Problems will arise if condensers and heat exchangers have been re-tubed with tubes of titanium or other corrosion-resistant materials but the original tube plates have to be retained. In this case, corrosion may easily arise in the tube plates. During retubing the new tubes have then usually been machined down so that their free ends are in the plane of the outside of the respective tube plate.